Hydrostatic or “hystat” drive refers generally to a drive train or portion of a drive train in a work machine utilizing hydraulic fluid pressurized by engine rotation as the motive force for propelling the work machine. In a typical design, a pump is driven with an output shaft of the engine and provides pressurized hydraulic fluid to a hydraulic motor, in turn coupled with one or more axles of the work machine. In most hystat drive work machines, the pump and motor each have a variable displacement, allowing the relative torque and speed of a drive shaft to be continuously varied.
Where it is desirable to provide a relatively high torque to the work machine wheels or tracks, the displacement of the motor will be relatively large such that at a given hydraulic pressure from the pump, a relatively large force is transferred to the drive shaft and wheels or tracks for each stroke of the motor. Similarly, where it desirable to provide a relatively lower torque to the work machine wheels or tracks, for example, when operating the work machine at relatively higher velocity, the relative displacement of the motor can be decreased. Displacement of the pump can be similarly adjusted, creating a continuously variable coupling between the engine and the ground engaging wheels or tracks of the work machine.
While the combination of a variable displacement pump and variable displacement motor in a hystat work machine imparts tremendous flexibility in operation, there is room for improvement. Particularly in work machine applications that require relatively frequent directional changes, for example, loader operations, the wear and tear on the work machine and even the operator can be significant.
Most if not all traditional hystat work machines require the operator to manually control one or more of the hydrostatic drive components. In many common hystat designs, the direction of fluid flow between the pump and motor is reversed to change the direction of motor rotation, and in turn change the direction of rotation of a drive shaft. In order to change fluid flow direction where the pump is manually controlled, the operator must decrease the fluid flow in the first flow direction, then increase fluid flow in the opposite direction. Wheel brakes, engine brakes, or other slowing means may be simultaneously employed to slow the work machine to a stop prior to reversing directions.
Reducing pump displacement, or “downstroking” the pump also provides a retarding force on the work machine. Increasing pump displacement, or “upstroking” the pump tends to accelerate the vehicle. Meanwhile, engine rotation continues to drive the pump, the engine speed being based predominantly on throttle position, also controlled by the operator, and a load on the engine. Accordingly, where an operator wishes to switch directions, he or she must attempt to integrate all of the various accelerating and decelerating forces applied to the work machine to execute a smooth and efficient directional change, often a challenging or impossible task.
It is generally desirable for an operator to perform a directional shift relatively quickly, simply to maximize the rate at which the work machine can move around the work environment and perform its various tasks. Thus, operators will typically attempt to slow the work machine, reverse fluid flow between the pump and motor, and accelerate as quickly as possible. However, the various hystat components, and the work machine itself are not without limitations.
Where an operator adjusts the pump or motor too quickly, he or she risks stalling or overspeeding the engine, or accelerating the work machine too quickly. Where the rate of work machine acceleration changes too quickly, the work machine can jerk. Overly large accelerations or changes in acceleration can be not only uncomfortable for the operator, but can risk spilling loads carried by the work machine.
Highly skilled operators can typically execute directional shifts relatively rapidly and smoothly. However, many modern hystat systems simply have too many variable components for a single operator to attend to both shift duration and shift smoothness without sacrificing one for the other. For instance, no skill level can enable an operator to optimally control a changing pump displacement, motor displacement, throttle position, etc., all simultaneously. In other words, even operators with many years of experience can find it difficult or impossible to consistently reverse travel directions in the work machine without jerking or overly accelerating the same, or overly extending shift duration to avoid excessive acceleration or jerk.
In an attempt to assist operators in performing directional shifts both smoothly and efficiently, designers have developed various systems wherein an electronic controller performs the adjustments of one or more of the components in the hydrostatic drive during shifting. While these systems have shown some improvements over earlier designs, the operator is still responsible for controlling certain of the components. The operator's control can thus still limit shifting speed, for example, if the operator adjusts the pump or motor too slowly, or compromise smoothness, for example, if the pump or motor is adjusted too quickly. Moreover, different operators can have different personal shifting preferences. While one operator may be willing to undergo significant jostling during a shift, if the shifting duration is minimized, other operators may be unwilling or unable to tolerate jerks or rapid accelerations.
Co-owned U.S. Pat. No. 5,624,339 shows a method for controlling shift points in a continuously variable transmission that includes a hydrostatic drive path or a combined hydrostatic and mechanical transmission drive path. The mechanical transmission includes a planetary summing arrangement that appears to allow for smooth shift without disruption of torque. Although this strategy and structure appears promising, there always remains room for improving upon the overall combination of work efficiency with rider comfort.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.